Online Certificate Status Protocol Explained

OCSP
Long Name:Online Certificate Status Protocol
Status:Proposed Standard
Year Started:[1]
Domain:Digital certificate

The Online Certificate Status Protocol (OCSP) is an Internet protocol used for obtaining the revocation status of an X.509 digital certificate.[2] It is described in RFC 6960 and is on the Internet standards track. It was created as an alternative to certificate revocation lists (CRL), specifically addressing certain problems associated with using CRLs in a public key infrastructure (PKI).[3] Messages communicated via OCSP are encoded in ASN.1 and are usually communicated over HTTP. The "request/response" nature of these messages leads to OCSP servers being termed OCSP responders.

Some web browsers (e.g., Firefox[4]) use OCSP to validate HTTPS certificates, while others have disabled it.[5] [6] Most OCSP revocation statuses on the Internet disappear soon after certificate expiration.[7]

Certificate authorities (CAs) were previously required by the CA/Browser Forum to provide OCSP service, but this requirement was removed in August 2023, instead making CRLs required again.[8] Let's Encrypt has announced their intention to end OCSP service as soon as possible, citing privacy concerns and operational simplicity.[9]

Comparison to CRLs

Basic PKI implementation

  1. Alice and Bob have public key certificates issued by Carol, the certificate authority (CA).
  2. Alice wishes to perform a transaction with Bob and sends him her public key certificate.
  3. Bob, concerned that Alice's private key may have been compromised, creates an 'OCSP request' that contains Alice's certificate serial number and sends it to Carol.
  4. Carol's OCSP responder reads the certificate serial number from Bob's request. The OCSP responder uses the certificate serial number to look up the revocation status of Alice's certificate. The OCSP responder looks in a CA database that Carol maintains. In this scenario, Carol's CA database is the only trusted location where a compromise to Alice's certificate would be recorded.
  5. Carol's OCSP responder confirms that Alice's certificate is still OK, and returns a signed, successful 'OCSP response' to Bob.
  6. Bob cryptographically verifies Carol's signed response. Bob has stored Carol's public key some time before this transaction. Bob uses Carol's public key to verify Carol's response.
  7. Bob completes the transaction with Alice.

Protocol details

An OCSP responder (a server typically run by the certificate issuer) may return a signed response signifying that the certificate specified in the request is 'good', 'revoked', or 'unknown'. If it cannot process the request, it may return an error code.

The OCSP request format supports additional extensions. This enables extensive customization to a particular PKI scheme.

OCSP can be vulnerable to replay attacks,[12] where a signed, 'good' response is captured by a malicious intermediary and replayed to the client at a later date after the subject certificate may have been revoked. OCSP allows a nonce to be included in the request that may be included in the corresponding response. Because of high load, most OCSP responders do not use the nonce extension to create a different response for each request, instead using presigned responses with a validity period of multiple days. Thus, the replay attack is a major threat to validation systems.

OCSP can support more than one level of CA. OCSP requests may be chained between peer responders to query the issuing CA appropriate for the subject certificate, with responders validating each other's responses against the root CA using their own OCSP requests.

An OCSP responder may be queried for revocation information by delegated path validation (DPV) servers. OCSP does not, by itself, perform any DPV of supplied certificates.

The key that signs a response need not be the same key that signed the certificate. The certificate's issuer may delegate another authority to be the OCSP responder. In this case, the responder's certificate (the one that is used to sign the response) must be issued by the issuer of the certificate in question, and must include a certain extension that marks it as an OCSP signing authority (more precisely, an extended key usage extension with the OID)

Privacy concerns

OCSP checking creates a privacy concern for some users, since it requires the client to contact a third party (albeit a party trusted by the client software vendor) to confirm certificate validity. OCSP stapling is a way to verify validity without disclosing browsing behavior to the CA.

Criticisms

OCSP-based revocation is not an effective technique to mitigate against the compromise of an HTTPS server's private key. An attacker who has compromised a server's private key typically needs to be in a man-in-the-middle position on the network to abuse that private key and impersonate a server. An attacker in such a position is also typically in a position to interfere with the client's OCSP queries. Because most clients will silently ignore OCSP if the query times out, OCSP is not a reliable means of mitigating HTTPS server key compromise.[13]

The MustStaple TLS extension in a certificate can require that the certificate be verified by a stapled OCSP response, mitigating this problem. OCSP also remains a valid defense against situations where the attacker is not a "man-in-the-middle" (code-signing or certificates issued in error).

The OCSP protocol assumes the requester has network access to connect to an appropriate OCSP responder. Some requesters may not be able to connect because their local network prohibits direct Internet access (a common practice for internal nodes in a data center). Forcing internal servers to connect to the Internet in order to use OCSP contributes to the de-perimeterisation trend. The OCSP stapling protocol is an alternative that allows servers to cache OCSP responses, which removes the need for the requestor to directly contact the OCSP responder.

Browser support

There is wide support for OCSP amongst most major browsers:

However, Google Chrome is an outlier. Google disabled OCSP checks by default in 2012, citing latency and privacy issues[20] and instead uses their own update mechanism to send revoked certificates to the browser.[21]

Implementations

Several open source and proprietary OCSP implementations exist, including fully featured servers and libraries for building custom applications. OCSP client support is built into many operating systems, web browsers, and other network software due to the popularity of HTTPS and the World Wide Web.

Server

Open source

Proprietary

Library

Open source

Client

See also

External links

Notes and References

  1. Web site: History for draft-ietf-pkix-rfc2560bis-20. June 2013 . December 23, 2021 . Santesson . Stefan . Myers . Michael . Ankney . Rich . Malpani . Ambarish . Galperin . Slava . Adams . Carlisle .
  2. Web site: How To Configure OCSP Stapling on Apache and Nginx . Digital Ocean, Inc. . Community Tutorials . June 12, 2014 . March 2, 2015 . A., Jesin.
  3. Web site: OCSP Stapling . GMO GlobalSign Inc. . GlobalSign Support . August 1, 2014 . March 2, 2015.
  4. Web site: CA/Revocation Checking in Firefox. wiki.mozilla.org. 29 June 2022.
  5. Web site: Are revoked certificates detected in Safari and Chrome?. 20 September 2017 . 29 June 2022.
  6. Web site: CRLSets. 29 June 2022.
  7. Korzhitskii . Nikita . Carlsson . Niklas . 2021 . Revocation Statuses on the Internet . Hohlfeld . Oliver . Lutu . Andra . Levin . Dave . PAM 2021 . Passive and Active Measurement . . 12671 . 175–191 . 2102.04288 . 10.1007/978-3-030-72582-2_11 . 978-3-030-72582-2 . 0302-9743.
  8. Web site: Barreira . Inigo . [Servercert-wg] IPR Review period for SC63: Make OCSP optional, require CRLs, and incentivize automation ]. lists.cabforum.org . September 28, 2023 . August 4, 2024.
  9. Web site: Aas . Josh . Intent to End OCSP Service . Let's Encrypt . July 23, 2024 . August 4, 2024.
  10. Web site: Security Certificate Revocation Awareness: The case for "OCSP Must-Staple" . Gibson Research Corporation . March 2, 2015 . Gibson, Steve.
  11. Web site: OCSP Stapling in Firefox . Mozilla Foundation . Mozilla Security Blog . July 29, 2013 . March 2, 2015 . Keeler, David.
  12. RFC 6960, section 5, Security Considerations
  13. Web site: No, Don't Enable Revocation Checking. 19 April 2014. 24 April 2014.
  14. Web site: Windows XP Certificate Status and Revocation Checking. Microsoft. 9 May 2016.
  15. Web site: What's New in Certificate Revocation in Windows Vista and Windows Server 2008. 3 July 2013 . Microsoft. 9 May 2016.
  16. Web site: Mozilla Bug 110161 – Enable OCSP by Default. Mozilla. 1 October 2007. 18 July 2010.
  17. Web site: Apple users left to defend themselves against certificate attacks. Wisniewski. Chester. Sophos. 26 March 2011. 26 March 2011.
  18. Web site: Introducing Extended Validation Certificates. Pettersen. Yngve Nysæter. November 9, 2006. Opera Software. 8 January 2010. dead. https://web.archive.org/web/20100210031759/http://labs.opera.com/news/2006/11/09/. 10 February 2010.
  19. Web site: Rootstore newsletter. Pettersen. Yngve Nysæter. 3 July 2008. Opera Software. 8 January 2010.
  20. Web site: Revocation checking and Chrome's CRL. 5 Feb 2012. 2015-01-30. https://web.archive.org/web/20120212014932/http://www.imperialviolet.org/. live. 2012-02-12. Langley, Adam.
  21. https://www.zdnet.com/article/chrome-does-certificate-revocation-better/ "Chrome does certificate revocation better"
  22. Web site: Boulder – an ACME CA. GitHub. 16 March 2018. 17 March 2018.
  23. Web site: Dogtag Certificate System. 12 Aug 2019.
  24. Web site: EJBCA – Open Source PKI Certificate Authority. PrimeKey. 2 February 2018. 17 March 2018.
  25. Web site: XiPKI. GitHub. 13 March 2018. 17 March 2018.
  26. Web site: OpenCA OCSP. 3 January 2024.
  27. Web site: Certificate Services (Windows). Windows Dev Center. Microsoft. 2018. 17 March 2018.
  28. Web site: Package ocsp. cfssl GoDoc. 25 February 2018. 17 March 2018.
  29. Web site: OCSP_response_status. master manpages. OpenSSL. 2017. 17 March 2018.
  30. Web site: OCSP in wolfSSL Embedded SSL – wolfSSL. 2014-01-27. en-US. 2019-01-25.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Online Certificate Status Protocol".

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